The present invention relates to an air fuel ratio detecting arrangement using a diffusion suppressing type air fuel ratio sensor applicable to an air fuel ratio control unit for an internal combustion engine and the method therefor, and, in particular, relates to an air fuel ratio detecting arrangement using a diffusion suppressing type air fuel ratio sensor applicable to an air fuel ratio control unit for an electronic fuel injection type internal combustion engine for a motor vehicle and the method therefor.
The output characteristics of a diffusion suppressing type air fuel ratio sensor varies during long term use because of clogging of the porous gas diffusion suppressing portion thereof through deposition of exhaust gas components and generation of cracks therein due to thermal shock caused by combustion gas. Further, the output characteristics of a diffusion suppressing type air ratio sensor varies depending upon atmospheric air pressure to cause errors in detected air fuel ratio, in such a way that an air fuel ratio set for the motor vehicle shifts toward a lean side to thereby cause the driving performance of the motor vehicle to deteriorate.
JP-A-62-79344(1987), which corresponds to U.S. Pat. No. 4,676,213, discloses calibration of output characteristics change of a diffusion suppressing type air fuel ratio sensor due to secular change of the diffusion layer wherein maximum outputs thereof are sampled and renewed by the latest one, and the output characteristic of the sensor is calibrated every time when the maximum output is renewed with the renewed maximum output so as to compensate for secular change of the diffusion layer. The term "secular change" is defined as the change in the characteristics and performance of a sensor caused by aging of the sensor through the passage of time.
JP-A-62-267544(1987) discloses the calibration of output characteristic change for a diffusion suppression type air fuel ratio sensor due to secular change of the diffusion layer wherein detection signals at a predetermined air fuel ratio other than stoichiometric air fuel ratio is read and compared with the value of the initial detection signal to calibrate the sensor output characteristic so as to compensate for secular change of the diffusion layer.
JP-A-2-102447(1990), which corresponds to U.S. Pat. No. 4,944,274, discloses the calibration of the output characteristic change of a diffusion suppressing type air fuel ratio sensor due to secular change of the diffusion layer wherein variation of a diffusion coefficient from an initial value is detected and the output characteristic of the sensor is calibrated based on the detected variation so as to compensate for secular change of the diffusion layer. In the above mentioned prior diffusion suppressing type air fuel ratio sensors, the calibration of the output characteristic change is principally determined based on secular change of the diffusion layer. Other parameters affecting the detection accuracy of such diffusion suppressing type air fuel ratio sensors are not sufficiently taken into account.
An object of the present invention is to provide an air fuel ratio detecting arrangement and method therefor for an internal combustion engine having a high detection accuracy over a long use term thereof.
According to one aspect of the present invention, the above object is achieved by incorporating in an air fuel ratio detecting arrangement an atmospheric air pressure correction means in a signal processing and outputting means for correcting an output voltage signal processed therein based on a change in atmospheric air pressure which is detected when a predetermined operating condition of an internal combustion engine is satisfied in addition to an air calibration means.
According to another aspect of the present invention the above object is achieved by incorporating in an air fuel ratio detecting arrangement a secular change correction means included in a signal processing and outputting means for detecting a change in electromotive force of a concentration cell portion of a diffusion suppressing type air fuel ratio sensor when a predetermined rich air fuel ratio is reached and for correcting a reference voltage for the diffusion suppressing type air fuel ratio sensor based on the detected change in electromotive force of the concentration cell portion in addition to an air calibration means.
According to a further aspect of the present invention, the above object is achieved by incorporating in an air fuel ratio detecting method the step of determining an atmospheric air pressure change when a predetermined operating condition of an internal combustion engine is satisfied, the step of correcting an output voltage signal obtained by processing a signal relating to an air fuel ratio from a diffusion suppressing type air fuel ratio sensor based on the determined atmospheric air pressure change, the step of determining a secular change in electromotive force of a concentration cell portion of the diffusion suppressing type air fuel ratio sensor when a predetermined rich air fuel ratio is reached and the step of correcting a reference voltage for the diffusion suppressing type air fuel ratio sensor based on the determined secular change in electromotive force of the concentration cell portion in addition to the step of calibrating the output voltage signal based on a calibration through atmospheric air.